1. Field of the Invention
The present invention relates to flat panel display systems, and more particularly concerns such a system employing a rear projection screen and having greatly improved brightness.
2. Description of Related Art
Flat panel display systems are commonly employed in so-called lap-top, notebook or other portable computers in which packaging dimensions are of critical significance. In particular, a major effort in the design and construction of such flat panel display systems is an attempt to keep the thickness of the display system to a minimum. Minimum display thickness or depth is obtained by employing a flat minimum depth illumination source. The illumination source often comprises a flat fluorescent light system and a contiguous diffuser that are positioned closely adjacent the rear surface of an active matrix liquid crystal module with a transparent screen or other protective layer covering the front face of the liquid crystal module. The diffuser is provided in an attempt to distribute light from the fluorescent source evenly over the liquid crystal module. In viewing the displayed image the viewer effectively looks through the liquid crystal module at the diffused light source behind it. This light generally has a Lambertian distribution, e.g. a uniform distribution in which intensity decreases with the cosine of the viewing angle away from an on-axis direction, a perpendicular to the screen.
A normal vertical viewing angle of an observer is plus or minus 15.degree., although horizontal viewing angles are generally larger. The diffused light passing through the liquid crystal panel and transmitted at a vertical angle outside of the plus or minus 15.degree. vertical viewing angle is effectively wasted. Utilizing fluorescent illumination and even with a diffuse light source behind the liquid crystal panel, it is difficult to provide a uniformity of brightness or light intensity over a large surface area of the display. Further, because the viewer is effectively looking through the liquid crystal display panel at the diffuse light source behind it, that portion of the light path from the viewer's eye through the liquid crystal material itself changes in length as viewing angle changes. That is, more of the liquid crystal material will act on a light ray passing through the liquid crystal panel at an angle than on a light ray passing through the liquid crystal panel at a normal to its surface. This results in degradation of image contrast because the greater the light path length through the liquid crystal material, the greater the effect of the liquid crystal material. Color is also degraded by use of a diffuse light source behind the liquid crystal module.
Because of the panel thickness limitation, high intensity light sources are not used, and overall screen brightness is relatively low. Such systems have very poor visibility in high ambient light and are nearly impossible to utilize in direct sunlight. Further, the dynamic range of available light intensity is limited so that a system arranged for operation in relatively high intensity ambient light cannot be operated with sufficiently low brightness level for night operation.
Accordingly, it is an object of the present invention to provide a flat panel liquid crystal display system that avoids or minimizes above mentioned problems.